The present invention relates generally to screw-threaded fasteners, and more particularly to self-tapping screw-threaded fasteners having first and second threaded portions axially separated by means of an unthreaded portion with the diameter of the second threaded portion being greater than that of the first threaded portion, and wherein a specially configured, hole-enlarging transition portion or zone is defined between the trailing end portion of the first threaded portion and the leading end portion of the unthreaded portion so as to permit passage of the second threaded portion through a component to be fastened to a support element.
Our earlier patent specification GB-A-1270545 describes and claims a screw threaded fastener which is particularly arranged for fixing a corrugated metal roofing sheet onto underlying metal purlins. The fastener includes a self-drilling point, a first self-tapping screw thread adjacent the drilling point, an unthreaded region, a head and a second screw threaded portion adjacent the head. In use, the drilling point firstly drills a hole within the corrugated sheet material and the first screw threaded portion forms a threaded region within the sheet metal component. Then, with the sheet metal component within the vicinity of the unthreaded portion of the fastener the drilling point drills a hole within the underlying purlin. Once this has been completed the first screw threaded portion cuts a thread within the underlying purlin and, as the fastener is screwed into the purlin the second screw thread engages the roofing sheet and urges it rearwardly so as to clamp it between the second screw threaded portion and the head of the fastener and of course any waterproof sealing washer interposed between the top of the roofing sheet and the head of the fastener. Such fasteners have been successful and used widely.
Our earlier patent specification GB-A-1277044 describes an improvement of the fastener described in GB-A-1270545 by fabricating the fastener by means of a process including two forward extrusion steps using three different punch tools so that the diameter of the fastener adjacent the head is greater than the diameter of the fastener adjacent the drill point. This enables the second screw threaded portion to have a larger diameter so that the crest diameter of the second screw thread is much greater than that of the first screw thread. This provides a more positive engagement of the roofing sheet with the second screw threaded portion and so forms a more reliable seal between the head of the fastener and the roofing sheet. In order to provide clearance for the larger diameter portion of the fastener the process also includes a subsequent forging step during which wings are formed upon the shank of the fastener upstream of the trailing edge portion of the first screw thread. The wings firstly ream out the screw thread formed withing the roofing sheet by the means of the first screw threaded portion and provide an enlarged hole which allows sufficient clearance for passage therethrough of the larger diameter portion of the fastener, and secondly, when and if they come into contact with the upper face of the purlin, they prevent overdriving of the fastener.
Forming the blanks for the fastener so as to form the portions of larger diameter and also the subsequent forging operation so as to produce the upstanding wings leads to a production process for these fasteners which is expensive with the result that while the fasteners are technically superior they are more expensive to manufacture which makes it more difficult for them to compete in the market place with fasteners made in accordance with patent specification 1270545.